ferris, you have two aircraft opposite direction. For simplicity's sake they are flying on separate airways that run parallel and actually join at the edges, each delineated by VORs. Effectively then we have a chunk of CAS, say 17 miles wide, with two parallel tracks 7 miles apart. Our two aircraft are on their own nav, one requiring climb ('cos it is a departure) and one requiring descent ('cos it is an inbound). At some point their levels will cross and all other things being equal they will be 7 miles apart laterally.
I grant you the arguement that before altering heading away from own nav the pilot should inform/ask ATC. I grant you also that if you had told both aircraft to "route direct" to whatever VOR marked the end of that airway then everything
should be OK. However, in aviation we all know that things don't always go according to plan. Congested frequency, CBs, first language not English, nav-aid failure (a/c or ground), misheard call-sign, you name it. The only
sure way of proving you did everything to ensure separation is to lock both aircraft on headings, and of course, listen to the read-backs. It may well be the headings you use are the ones they were already on - great. That makes "resume own nav" dead easy when the crisis, sorry, confliction is past.
Whether this is logical, sensible or totally over the top I care not. It is what was taught
When I wrote this I was trying to separate (pun unintended) the rationale of the method from the immediate reason why it is used. It is used because controllers are taught it.
The rationale as to why it is taught is as above. Leaving pilots to do their own thing proves nothing, other than the controller has "assumed" that all will be well. He has done nothing to "ensure" separation. And at the subsequent Court of Enquiry, if you did not
prove the separation then the possibility of a manslaughter charge is on the cards. The cynical but realistic person could describe it as an arse-covering exercise. Experience teaches to watch your six o'clock because no-one else will.
Oh, and by the way ferris, I never was a manager - I thought too much!